Porous fluoropolymer films have been used in a wide variety of applications, including biomedical applications wherein fluoropolymer film patch grafts are surgically anastomosed into an existing organ or tissue for surgical repair or reconfiguration thereof. Synthetic patch grafts of this type are typically used to repair various anatomical structures and organs, including blood vessels, heart, skin, soft tissue, pericardium, etc.
One fluoropolymer commonly used for the manufacture of porous films is polytetrafluoroethelyne (hereinafter "PTFE"). PTFE has excellent heat resistance, chemical resistance, self-lubricity, non-adhesiveness and biological compatibility. As a result of these desirable properties, PTFE porous films have found wide applicability in medical, industrial and other applications.
A basic method for manufacturing porous PTFE films is described in U.S. Pat. No. 4,478,665 (Gore). In accordance with this basic method, a PTFE paste is prepared by mixing crystalline PTFE fine powder with a quantity of liquid lubricant. The paste is subsequently extruded and calendared to form a wet, unsintered film extrudate. The film extrudate is cut into components. The components of PTFE containing a liquid lubricant are placed in intimate contact. The film extrudate is subsequently dried, expanded in at least one axis, and sintered. The sintering process is carried out by heating the PTFE to a temperature above its crystalline melting point (327.degree. C.) but below the thermal degradation temperature thereof, for sufficient time to cause the PTFE polymer to substantially convert from its crystalline state to an amorphus state. In this regard, the sintering of PTFE is sometimes referred to as "amorphus locking" of the polymer.
Expanded, sintered PTFE films manufactured by the above-described basic process have a microstructure characterized by the existence of relatively dense areas known as "nodes" interconnected by elongate fibrils. The strength and porosity of the sintered PTFE film is largely a function of the directional orientation and spacing between the microstructural fibrils.
The directional orientation of the microstructural fibrils is determined by the directional axis or axes in which the film is a) calendared and b) expanded, prior to sintering thereof. Sintered PTFE films which have been calendared and expanded uniaxially typically have high strength only in the direction of the axis in which the film was calendared and expanded. Similarly, PTFE films which have been biaxially calendared and expanded may subsequently have high strength in both axes in which the film was previously calendared and expanded.
It is desirable to develop methods for manufacturing multiaxially calendared and expanded films which will exhibit substantially isotropic strength properties in all directions. Such multiaxially oriented films may exhibit highly uniform strength properties in all directions, thereby providing superior films for use in applications, such as biomedical patch graft applications, wherein multiaxial orientation and isotropic strength properties are desirable.
Prior efforts to manufacture multi axially oriented PTFE films have been described. For example, U.S. Pat. No. 4,478,655 (Hubis) purports to describe a method for producing a composite or "multicomponent" porous PTFE film wherein a plurality of individual uniaxially oriented films are placed in juxtaposition, in varying orientations, and subsequently fused or laminated to one another to produce a composite article which exhibits composite multi axial orientation and isotropic strength properties.
There remains a need in the art for the development of new and/or improved methods for manufacturing thin, porous fluoropolymer films having multi axial fibril orientation and resultant isotropic strength properties after sintering.